This invention relates generally to computer communication networks and, more particularly, to a repeater circuit for a CAN-based network which provides the ability to identify network branch faults as well as to electrically isolate faulty branches from the remainder of the network.
Computer communication networks generally provide interconnection among a number of independent computing stations or devices within a defined area such as a building or plant. At higher data rates, these networks typically employ a topology wherein a long linear bus has a number of relatively short branches connected thereto in order to reduce the signal reflections which would occur due to the impedance mismatch of a long branch. Each branch has one or more computers or devices connected thereto, and a signal transmitted from any station on any branch in the network propagates the length of the bus and can be received by any one or all of the other stations. Signals transmitted in this type of network are generally in the form of "packets" of logically high or low bits which include information such as the address of the transmitting and destination stations used in a manner well known to those in the art to achieve message transfer.
In network applications where interconnected devices are physically distant, such as in an industrial control environment where connected devices may be spread over a large manufacturing plant floor, it takes a significant amount of additional cable to route the linear bus to each device to achieve short branch lengths. However, if relatively long branch lines were used instead to connect all of the devices in order to reduce the amount of cable required, significant degradation of the transmitted signals would occur due to signal reflection. One way of reducing the restricting effects caused by signal degradation and reflection on network topology, while maintaining sufficient signal integrity, is through the use of repeaters. These devices amplify and reconfigure a transmitted signal as it passes therethrough. However, this also means that faults and various signal distortions are amplified as well, thereby corrupting signal transmission on other parts of the network.
The circuit of the present invention provides an improved repeater circuit for a network operated in conformance with a Controller Area Network (CAN) protocol. CAN is a serial communication protocol which supports distributed real time control and multiplexing using a differentially driven 2-wire bus line with a common return as a physical medium. In addition to providing traditional repeater functionality, the present repeater circuit includes isolation circuitry for controllably disabling the repeater, and thereby preventing the further transmission of a fault-indicative signal on the network, such as may be caused by a signal transmission line being shorted to power or ground. Based on the required structure of messages to be transmitted on a CAN-based network, an error or fault in the signal is assumed whenever the signal is dominant (low) for more than 13 bit times.
The present isolation circuit selectively disables the passing of such a fault-indicating signal to the receive input of the repeater circuit. A retriggerable one-shot starts timing with each signal transition from a high to a low logic level. If a second such high to low transition does not occur within a time period equivalent to 13 bit times, the one-shot sends a pulse to a resettable flip-flop which is tripped only if the input signal is low. In response to being tripped, the flip-flop produces an output signal which disables the transmission of the signal to the repeater through a logic gate.
Therefore, while the repeater circuit itself provides the benefits of a traditional repeater as discussed above, the fault isolation circuitry additionally provides the ability to identify a signal fault and isolate a segment transmitting a faulty signal from the remainder of the network. This allows the network to function independently of the faulty segment. In addition, the first transition passes through the isolation circuitry without the additional delay induced by the one-shot circuit thereby providing for fault isolation with a minimal effect on signal propagation time. The repeater and associated fault isolation circuitry is small enough to fit inside the confines of a "tee" shaped or in-line connector, used to physically as well as electrically join two segments of cable.
These and other features and advantages of the present invention will become apparent upon review of the following description, taken in conjunction with the accompanying drawings.